Wave filter



April 30, 1940. R. AfsYKEs 2,198,684

WAVE FILTER Filed Sept. 20, 1938 2 Sheets-Sheet 1 ATTENUAT/ON 2 4 s msamvcr C T D A s k 5 E k a FIG. 8 g o I; 24 1; 3 FREQUENCY E lNl/ENRDR R. A. 57/655 ZFREOZENCY a ATTORNEY R. A. s YKEs WAVE FILTER Filed Sept. 20, 1938 ATTORNEY f fl: /a f/s FREQUENCY FREQUENCY FIG. /6

f2! 22 fa fzs FmuE/vcr FIG. /7

ri -game? T wvmron R. ASV/(ES A BY Patented Apr. 30, 1940 UNITED STATES PATE NT OFFICE WAVE FILTER Roger A. Sykes, Fanwood, N. J., assignor toBell Telephone Laboratories,

Incorporated, New

York, N. Y, a corporation of New York Application September 20, 1938, Serial No. 230,775

25 Claims.

ticularly to wave filters of the unbalanced type.

Anobject of the invention is to increase the width of the transmission band in an unbalanced band-pass wave filter which uses piezoelectric crystals as impedance elements.

Another object is to provide a wave filter of this type in which the attenuation peaks may be located anywhere in the frequency spectrum, including regions far removed from thetransmission band. i j

A further object is to increase the height of the peaks of attenuation.

Another object of the invention is to provide filter circuits of this type which have, respectively, inherently high and low image impedances.

The wave filter of the present invention is of the unbalanced, band-pass type and makes use of a single piezoelectric crystal having divided electrodes on both sides. Two of the electrodes are connected together and to the grounded side of the filter. The other two electrodes are connected, respectively, to aninput terminal and the corresponding output terminal onthe high side of the filter. Different typesof attenuation characteristics may be obtained depending upon whether the two connected electrodes are on the same side of the crystal or on opposite sides. If

the connectedelectrodes are on the same side they may be replaced by a single electrode. A bridging impedance branch including a capacitor is introduced between the two unconnected electrodes to change the location of the attenuation peaks.

. Equal capacitors may be connected in shunt at the end of the crystal to decrease the width of the transmission band.

In order to widen the transmission band two equal inductors are added to the filter circuit. For the higher frequency ranges these inductors may be sections of coaxial or shielded transmission line. If a filter having an inherently high image impedance is required the inductors are connected in shunt at the ends of the crystal, and if a low image impedance is desired they are connected in series with the crystal. In order to provide a greater variety of transmission characteristics relationship. By properly choosing the poling of the connections to the crystal electrodes, the loca- 1 tion and the poling of the inductors and themeflicient of coupling any one of a great variety These include a band-pass filter having two peaks of attenuation which may be placed both on the lower side of the band, both on the upper side, or one below and one above,.and certain of the peaks may be made to occur at zero or infinite frequency.

.In order to increase the height of the attenuation peaks a balancing resistance may be added to the filter. The resistance is connected either in parallel with the bridging branch or between acommon terminal of the shunt inductors and the grounded side ofthe filter. Theproper value of this resistance depends upon the effective resistance of the inductors and the coefiicient of coupling, and its function is to maintain a resistive balance, at the peak frequencies, between the line and diagonal branches of the equivalent lattice network.

The nature of the invention will be more fully understood from the following detailed description and by reference to the accompanying drawings, of which:

Fig. 1 is a schematic circuit of one form of the wave filter in accordance with the invention, in which the end inductors are connected in shunt and the cross-connection tothe crystal electrodes is used;

Fig. 2 is a perspective View partly broken away showing how the electrodes are placed upon the piezoelectric crystal element;

Fig. 3 shows the equivalent lattice network for the filter of Fig. 1;

Fig. 4 represents the reactance-frequency characteristics of the impedance branches of the lattice network of Fig. 3;

Figs.j 5 and 6 represent typical attenuation characteristics for the filter of Figs. 1 and 3;

. Fig. 7 shows the other poling for the connectio-ns to the electrodes of the crystal element in Fig. 1; Fig. 8 represents a typical attenuation characteristic obtainable with the filter circuit of Fig.. 7;. v 1

Fig. 9 shows a modification of the filter circuit of Fig. 7 in which the inductors are connected l series aiding;

Fig. 10 is the equivalent lattice network for the filter of Fig. 9;

Fig. 11 shows the reactance-frequency characteristics of the impedance branches of the lattice network of Fig. 10;

Fig. 12 represents a typical attenuation characteristic for the filter of Figs. 9 and 10; of attenuation characteristics may be provided.

Fig. .13 shows another embodiment of the in Fig. 19 represents a typical attenuationc han' acteristic obtainable with the filter of Fig. 18.

Fig. l is a schematic circuit of one form of the wave filter in accordance with the. invention employing shunt inductors and a cross-connected crystal. The filter is a symmetrical four-terminal network having a pair of input terminals l, 2 and a pair of output terminals 3, 4 to which terminal loads 'of'suitable impedance may be connected. The network is unbalanced in structure so that the path connecting terminals 2 and 4 may be grounded or otherwise fixed in potential. Thepath connecting terminals I and 3 may be termed the high side of the network. The filter comp-rises a piezoelectric crystal X1 provided with two electrodes '5, 6 on one of the major faces and two oppositely disposed electrodes 1, 8 on the op po'si'teiace. The two'diagonally opposite electrodes Sand 1 are connected to the grounded side of the filter, and the remaining electrodes 5 and '8 are connected, respectively, tothe input terminal 5 and the output terminal 3 on the high side ofthe network. The bridging branch comprising the capacitor C1 is connected between theelectrodes 5 and 8, and the two equal capaci tors C2, C2 are connected in shunt at the ends of the crystal. The two equal shunt inductors LiandLi 'arefconnected'at their upper ends to the respective ends of the bridgingbran'ch and their other ends are connected together and through thefv'ariable resistor R to thegrounded [side or the filter. flf the'transmission. band of the "filter is centered in the higher fre'quency ranges, these inductorsmay advantageously be formed of sections of coaxial or shielded uniform transmissionline. [In other circumstances they "may be ordinary coil windings with cores of either magnetic ornon magnetic material. The inductors are inductively coupled by a mutual inductance, equal to KiLi where K; represents thecoefiicientof coupling. Due to the presence of the shunt inductors "this circuit has an inherently high image impedance. Fig.1 shows the unbalanced form of the filter but it may, of course, be built. as 'abalanced structure.

I The crystal element X1 is preferably of quartz in the "form of a relativelynarrow rectangular plate cut perpendicular to the electrical axis of 'themother crystal and with its length either in thedirection'of the mechanical axis or making a selected acute angle therewith. .Such a crystal will vibrate longitudinally when alternating po-' tentials are applied'to electrodes placed on the larger surfaces. Other well-known types of crys tal cut may be used and, under certain conditions, they may be preferred. The crystal shown in Fig.1 is of the rectangular type described above but for convenience is shown in end elevation.

The placing of the electrodes on thecrystal Xi is shown more clearly in the perspective View of Fig. 2. A corner of the crystal has been broken away to expose a portionof one of theelectrodes "8 on the underside of 'thecrystal. 'The elec- -along the optical axis.

trodes may be of silver, aluminum, or other suitable metal, plated directly onto the crystal, and may be applied by plating the two major surfaces all over and afterwards removing a narrow longitudinal strip of the plating along the center of each face. It is'generally desirable also to remove narrow strips of plating around the edges of the crystal. When the crystal vibrates in the longitudinal mode it is preferably supported between one or more pairs of oppositely disposed points or knife-edge clamps which contactthe crystal in the nodal region near the center and Connections to the electrodes may be through these clamps or by attaching leads directly to the electrodes with soft solder.

Since the network of Fig. 1 is symmetrical with respect to its input terminals I, 2 and its output terminals 3, 4 its properties may be investigated most conveniently from a consideration of the symmetrical lattice network to which it is equivalent, Each line branch of "the equivalent lattice is equal to half of the impedance measured between the high "side terminals i and 3 of Fig. 1, and each diagonal branch is equal to twice the impedance fmeasured between terminals 1 and 3 strapped together and the grounded side, t t is, terminal 2 a "4. It is apparent that the mechanical vibration of the crystal occurs for only one of these measurements, depending upon the poling of the connections to the crystal electrodes. Therefore, the impedance representing the piezoelectric properties or the crystal will'appear'in'only one branch of the lattice. The electrode capacitance of the crystal, however, will appearinboth branches. Fig.3 shows the equivalent lattice for the poling of the electrodes shown in Fig. l, where theinterconne'cted electrodes are diagonallyopposite to each other. For this case the crystal impedance'appears in the line branch. It is assumed that the inductors Li and L1 are connected in the series-opposing relationship.

'In Fig; 3 the'cryst'al impedance is represented by its 'equi'val'entelectrical circuit comprising a capacitance Co shunted by a branch consisting 'ofanin'ductance L in series witha second-capacitance C. The capacitance Co represents the simple electrostatic capacitance between a pair of 'oppo'sitely'disposed electrodes, such as 5 and i. The values of the capacitance Cand the in- 'ductan'ce'Ldepend upon the dimensions of the crystal and upon its piezoelectric and elastic con- "stants, These elements 'may be evaluated, in

terms ofj the dimensions of the crystal X1, from the following formulas, assuming that the crystalfis ofthe X-cut variety described above and that theelectrodes cover substantially the entire area of the two majorfaces:

jp' iises two similar line impedance branches Z1" and 'two' similar --uiagona1 impedance "branches Z2.

Each line branch is made upot three parallel arms, one consisting, of an inductance equal to (1-K1).L1 in series with a .resultance R1 representing the. effectiveyresistance associated with the inductor L1,. the second consisting of the inductance Lin series with the capacitance C, and

the third being acapacitance equal in magnitude to. the sum of Co, 201 andC2. Each diagonal 9 branch Z2 is made upof acapacitance equal to the sum of and C2 shunted-by an arm consisting of aninductance equalto (1 +K1)L1in series cwith a resistance equal to the sum of R1 and 2R3. For the sake of clarity, in this figureand also in Figs. 10 and 14 onlyone line branch and .onediagonal branch are shown in detail, the other correspondingline anddiagonal branches being indicated by dotted lines connecting the appropriate terminals. 11 1 The image impedanceZx of the latticenetwork .ofFig. 3 isgiven in terms of the imped-ances of the line and diagonal branches by the expression x=1/ 1 2 and the propagation constant Pmay be found fromthe expression 1 1 f; 1. tanh t The filter will have a transmission band in the region where Zrand Z2 are of opposite sign and will have attenuation bands where Z1 and Z2 are .of thesame sign, with peaks of attenuation occurring where these impedances are equal. By

1 virtue of the equivalence of the two networks these expressions also give the impedance and propagation constant of the filter of Fig. l. i 1 Fig. 4 gives the reactance-frequency characteristics of the line and. diagonal branches of the lattice network ofFig. 3. As shown by the solid line curve the line branch Z1 has anti-resonances at the frequencies f2 and f4 witha resonance at the intermediate frequency is. The diagonal branch Z2 has a single anti-resonance, as shown by the dotted-line curve, and if this is made to coincide with the resonance of the line branch at is a band-pass filter will result. The transthisregion the reactances Z1 and Z2 are of 0pp0-. sitesign. Outside of this band the filter will at? tenuate since the reactances are of the same sign. If the inductors L1 and L1 are connected in the mission bandwill extend from ,fzto f4 because in series-opposing relationship the reactance characteris'tics of Z1. and Z2 will cross at some frequency f1 below the band and at another frequency ft above the band, as shown in Fig. 4.

These crossing points will determine the location of the peaks of attenuation. A typical attenuation characteristic is shown in Fig. 5. 1

.The .coefiicient of coupling K1 is found from ja kins mentioned; above. thefun ction. of t ad resistor Ra in Fig. 1 is to provide a resistive balance at the peak frequencies for the line and diagonal branches of the equivalent lattice of Fig. 3 in orderto increase the height of the attenuation peaks. The required value of R3 may be found from the formula, 1 1

. .(l-fKfi where R1 is the effective resistance oi theinductor L1 at some chosen frequency, say fa... The

resistor R3 may be made variable, as indicated by the arrow, in order to facilitate its adjustment to the proper value. When R is chosen in this way, the latticeof Fig. 3 will have the attenuation characteristic of a network made up of dissipationless elements with a resistance R0 shuntingthe input terminals I, 2 and a second repermissible ratio ofthe .reactance of the inductor L1 to itseffectiveresistance R is decreased. This means that for. the wider hands a less eflicient inductance 1 coil maybe used.

f The values oi the various reactance elements in the lattice of Fig. 3, including the electrical elements equivalent to the crystal, can be found from the resonant and anti-resonant frequencies ,of the Z1 and Z2 branches by a direct application of vR. M. Fosters reactance theorem giveninthe 1Bel1. System TechnicalJournal, vol. III, No. 2, April 1928, pages 259 to 267. The values of the .componentelements in the network of Fig. l are foundby applying the numerical factors indicated. 9 For the filter shown in Fig. l and Fig. 3 the values of the elements are given by the followingin whichxZis the design impedance of the filter and is usually taken as the value of the image impedance at the mid-band frequency f3.

'Ihechief purpose of the capacitor C1 is to determine the location of the attenuation peaks and the capacitor may be made variable, as indicated, for small adjustments of these peaks. If this capacitor is omitted the upper peak will ,occur at infinite frequency. As the value of C1 is increased the upper peak is moved to a lower If K1. is zero the lower peak As the band width of the filter is increased the .sistance shunting the output terminals 3, 4

The value of this resistance is given by t me filter using a piezoelectric crystalin'which the lower peak may be l'ocated at any frequency'lo'etween zero and the lo'wer cut-off and the upper peak niay be placed at any frequency [between 5 the upper cut-off and a high frequency which approaches infinity as the limit. The end capacitors C2, C2 have their chief efiect upon the width of the transmission band and may be made variable, as shown, if desired. The widest band 19 "is obtained when these capacitors are omitted nt e In '1 if the connections tothe "electrodes 6 and 8, for example, are interchanged, the circuit shown in Fig. 7 will result. Since the two 13 electrodes 1 and '8 on the one side of the crystal xz are now connected together, they maybe replaced 'by a single electrode 9 as shown. Changing the poling of the connections to the I electrodes has'the effect of removing the army consisting of the inductance Land the capacitance C from the line branch Z1 of the equivalent lattice shown in Fig. 3 and "placing it in parallel with the other arms in the diagonal branch Z2. The other'coinponent elements in the equivalent latticewill .be the same as shownin Fig. 3.

I fjthe inductorsL; and L1 are uncoupled, the

high-frequency 'pe ak willbe transferred to the lower side of the band while the peak at zero remains. Fig. 8 shows a typical attenuation 'ch'aracte'ristic, with peaks at. zero and f7 and a transmission band f-fr'om is to is.

in the circuit of Fig. 7 if the connection of the inductors is changed to the seriesaiding relaw "tionship'the'shunt-balancing resistonRz'may be omitted, but a resistor R4 connected. in parallel with the bridging capacitorciwill, in general,

be required. The'circuit'is shown'in Fig. 9. The

V MUG Of Ri' IIIBIY be foundirom the formula,

emoeroece'zrm 4 {LX112} in whichaw is 21r times the frequency at'which a resistive balance'is desired. The equivalent lattice network is given in 'Fig. 10 and the re-, actance characteristics of the'line and. diagonal impedance branches in Fig. 11. As shown by the typical attenuation characteristic in Fig. 12 there "will be "two attenuation peaks at in and f12 on the lower side of the transmission band, which extends from f 13 130 f15.

In placing the balancing resistor R3 in the path connecting thecommon terminal of the shunt inductors L1, L and thegrounded side of the filter as shown in Figs. 1 and '7, it is assumed that the core losses in the coils are low, such as are found in air-core coils, If inductors having magnetic cores, such as iron, are used it may be that the core losses are more than suflicient 5 supply, in effect, theresistancejRe. In this to ease the-resistive balance atthe-peak frequencies may be restoredlo'y the addition ofJa balancing resistor --in parallel with the {bridging branch, connected as is R4 in 9.

-In order to provide a' filter h'aving an inherm5 'ritl w image --im'pedance "the inductors are eonneaed in series on the high side at the'ends of the network,-as=shown inFig. 13. In this figure and the inter-connected electrodes are on the same side of the crystal X3 and are replaced 7o foya sing-leelectrodet. "The bridgingcapacitor C3 is connected between the two ele'ctrodes- 5 "and 6,- andthe equal capacitorsC4, C4 are connected in' shu'nt at the'ends of'the crystal. In this-case no balancing resistor willordin'arily-be v. equired.

result.

aieepsa.

ln Fig. I3 ifthe inductors am'i 2mmductively coupled seriesopposing :with :a :coefficient of coupling K2 the equival'entlatticepwill he asshown in' Fi'g. f14 and the reactance-irequen'c'y charact'eristics of theimpedance branches "will be of the typeishown -in-Fig. 15. The-sdia'gon'al branch 22 hasa resonanceiat the frequency '7'22;a second resonance*atjf24 and an intermediate anti-resonance at 723,= asshown :by the dottedline curve. The 1inebranchZ 1 -'has a'tsingle resonance,

as shown'by thesolid line curve, and if ithis is madetoccin'cide with the anti-resonance'ofthe -diagonal brahch '"at 23 .a band passi-filter-will istic of Fig.v 16 the transmission :bandwilli extend from 1]"22 to I24 and peaks of attenuation willv occur 'at 721 on the lowerside of the band "and at I25 on the upper side, where the two irea'ctance curv'escross. T

In Fig. l3"the coeflicient of coupling K2 may be found 'rr'om thj'e'forniula, T

The values of "the componentele'ments mayfibe found 'from the followingformulasz harem(Isa-mm lL2' inFig; 13"" is progressively decreas'ed the upper attenuation peak occurring'at I25 is' moved -to a l higher and higher frequency until, when K2 is As shown by the attenuation "characterequal to zero, the peak is re'legated to infinite frequency, as shown' in Fig. 17. At the same time the peak on the lower side of the bandis moved down to some lower frequency. 20. zero and a cross connected .crystal X4 is used,

as shown in Fig." 1 8-, the' 'peak'below the band will be transferred to some frequency fzs on'the upper side of the band, as shown in Fig. 19.

at infinity wilLremain \as before.

What is claimed is :i n 1. A wave filtercomprising'a pair of input terminals, a pair-ofv output terminals, a bridging impedance branch'including 'a capacitor. connecte'd between an input iterminaland a. co'rre- If K2 is kept Theipeak trodes associated with one face and two other oppositely disposed electrodes associated with the opposite face, two of said electrodes being connected respectively to the terminals of said bridgh in which said inductors are inductively coupled ingbranch. the remaining electrodes being connected to the remaining filter terminals, said inductors being connected at the respective ends of said crystal, the impedance measured between said first-mentioned input and output terminals having a different reactance-frequency characteristic from that of the impedance measured between said first-mentioned input and output terminals strapped together and the remaining terminals strapped together, and said two measuredimpedances being proportioned with respect to each other and with respect to two pre assigned frequencies so that said filter will freely transmita band lying between said frequencies while attenuating other frequencies.

. 2. A wave-filter in accordance with claim 1 in whichsaid' inductors are connected in series with said crystal. 3. A wave filter in accordance with claim 1 in which said inductors are connected respectively.

between the terminals ofsaid bridging branch and said remaining filter terminals. 1

4. A wave filter in accordancewith claim 1 in which said inductors are inductively coupled.

5. A wave filter in accordance with claim 1 in which said inductors are inductively coupled in the seriesaiding relationship. 1

6. A wave filter in accordance with claim 1 in which said inductors are inductively coupled in the series-opposing relationship.

'7. A wave filter in accordance with claim 1 which includes a resistor to increase the height of an attenuation peak, said inductors being connected respectively between the terminals of said bridging branch and one end of said resistor, and the other end of said resistor being connected to said remaining filter terminals.

8. A wave filter in. accordance with claim 1 which includes a resistor connected in parallel with said bridging branch to increase the height of an attenuation peak. i r 9. A wave filter in accordance with claim 1 in which said two interconnected electrodes are associated with the same face of said crystal.

10. A wave filter in. accordance with claim 1 in which said two interconnected electrodes are associated with opposite faces of said crystal and are diagonally opposite to each other.

11. A wave filter in accordance with claim 1 which includes two capacitors connected in shunt at the respective ends of said crystal.

12. A wave filter comprising a pair of input terminals, a pair. of output terminals, a bridging impedance branch including a capacitor connected between an input terminal and a correspondingoutput terminal, a piezoelectric crystal and two inductors, said crystal having two electrodes associated with one face and two other oppositely disposed electrodes associated with the opposite face, an electrode on one face and a diagonally opposite electrode on the opposite face being connected respectively to the terminals of said bridging branch, the remaining electrodes being connected to the remaining filter terminals, said inductors having a common terminal connected to said remaining filter terminals, the other ends of said inductors being connected respectively to the terminals of said bridging branch, and the dimensions of said crystal and thefvalues of 'said'capacitor'and said inductors being proportioned with respect to each other and withrespect to a pair of preassigned frequencies' to provide a transmission band between said frequencies.

13. A wave filter in accordance with claim 12 which includes a resistor in the path connecting the common terminal of saidinductors to said remaining filter terminals to increase the height of an attenuation peak. i 7 i 16. A wave filter comprising a pair of input terminals, a pair of output terminals, a bridging impedance branch including a capacitor connected between an input terminal and a corre-' sponding output. terminal, a piezoelectric crystal and two inductors, said crystal having a pair of electrodes associated with one face and a single electrode associated with the opposite face,-said pair of electrodes being connected respectively to the terminals of said bridging branch, said single electrode being connected to the remaining filter terminals, said inductors having a common terminal connected to said remaining filter terminals, the other ends of said inductors being connected respectively to the terminals of said bridging branch, and the dimensions of said crystal andthe valuesof saidcapacitorand said inductors being proportioned with respect to each other and withrespect to a pair of preassigned frequencies to provide a transmission band between said frequencies.

17. A wave filter in accordance with claim 16 in which said inductors are inductively coupled,

in the series-aiding relationship.

18. A wave filter in accordance with claim 16 which includes two capacitors connected in shunt at the respective ends of said crystal.

19. A wave filter in accordance with claim 16 which includes a resistor connected in parallel with said bridging branch to increase the height of an attenuation peak.

20. A wave filter comprising 'a pair of input terminals, a pair of output terminals, a piezoelectric crystal, two inductors and a bridging impedance branch including a capacitor, said crystal having a single electrode associated with one face and a pair of electrodes associated with the oppositeface, said single electrode being connected to an input terminal and a corresponding output terminal, one of said inductors being connected between the remaining input terminal and one of said pair of electrodes, the other of said inductors being connected between the remaining output terminal and the other of said pair of electrodes, saidbridging branch being connected between said pair of electrodes, and the dimensions of said crystal and the values of said inductors and said capacitor being proportioned with respect to each other and with respect to two preassigned frequencies to provide a transmission band between said frequencies.

21. A wave filter in accordance with claim 20 in which said inductors are inductively coupled in the series-opposing relationship.

22. A wave filter in accordance with claim 20 which includes two capacitors connected in shunt at the respective ends of said crystal.

23. A wave filter comprising. a pair of input terminals, a pair of output terminals, a piezotors being connected in shunt at the respective ends of said crystal, said third capacitor being connected between said remaining electrodes,

said inductors being inductively coupled in the series-opposing relationship and connected respectively between one of said remaining electrodes and said first-mentionedinput and output terminals, and the dimensions of said crystal and the values of. said inductors and said capacitors beingproportioned with respect to one another and. with respect to two preassigned. frequencies to provide a transmission band between said frequencies. a

24. A wave filter comprising: a pair of input terminals, a pair of output terminals, a piezoelectric crystal, two inductors, a pair of. capacitors'and' a third capacitor, said crystal having, a single electrode associated with one face and a pair of electrodes associated with the opposite face, said single-electrode being connected to an input terminal and a corresponding output terminal, said pair of electrodes being connected respectively to the remaining, terminals, said inductors being connected respectively between one of said pair of electrodes and said. first-mentioned input and output terminals, said pair of capacitors being connected in shunt at the respective ends of said crystal, said third capacitor being connected between said pair of electrodes, and s the dimensions of said crystal and the values of saidinductors and said capacitors being proportioned with respect to one-another and with respect to two preassignedfrequencies to provide a transmission hand between said frequencies.

25. A wave filter comprising a pair of input terminals, a pair of output terminals, a piezoelectric crystal, two inductors, a pair of capacishunt at the respective endsof said crystal, said third capacitor being connected between said pairsof electrodes, and the dimensions of said crystal and the values of said inductors and said capacitors being proportioned with respect to one another and with respect to two preassigned fre quencies to provide a transmission band between said frequencies. a

ROG-ER A. SYKES. 

